Sequences homologous to the highly repeated alpha-component DNA of the African green monkey occur interspersed among less highly reiterated monkey DNA sequences as well as in the long tandem repeats that were previously characterized. Homologous sequences also exist in the highly repeated DNA of the baboon where the repeat length of the alpha-component monomer (172 base pairs) is highly conserved but the organization of the sequences differs markedly. The segments that are highly homologous to alpha-component alternate, in the baboon genome, with another DNA segment of approximately the same length that is also related to alpha-component in sequence. Homologous sequences also exist, albeit in very low copy number, in the mouse genome. Monomeric and dimeric units of the tandem repeats of alpha-component from the African green monkey have been cloned in E.coli. In order to have a reliable and sensitive method for screening colonies of E.coli X1776 for foreign DNA fragments, a modification of published procedures, using lysozyme and triton, was devised.